This invention relates to bending magnets and more particularly to a bending magnet suitable for use in a synchrotron adapted to generate a synchrotron radiation (SR) or in a storage ring.
The SR is an electromagnetic wave which radiates from an electron e moving at a velocity approximating the velocity of light when the orbit of the electron is bent by a magnetic field H and because of strong directivity which is tangential to the orbit, the SR has many applications including, for example, a very effective use as a soft X-ray source for transfer of fine patterns of electronic parts.
The bending magnet is used to generate the magnetic field H which bends the orbit of the electron e for the sake of obtaining the SR.
As an example of the bending magnet, a superconducting bending magnet for use in a charged particle accelerator is disclosed in Japanese patent unexamined publication JP-A-61-80800. This example intends to generate a strong magnetic field of about 3 teslas, and has an iron core having upper and lower magnetic poles and upper and lower superconducting coils wound on the upper and lower poles, respectively. When the vertical distance between coil segments of the upper and lower coils disposed in the inner side of the orbit is h.sub.1 and the distance between the coil segments of the upper and lower coils disposed in the outer side of the orbit is h.sub.2, the bending magnet is divided into three areas in the direction of the orbit of charged particle beam and the superconducting coils are disposed such that the vertical distances h.sub.1 and h.sub.2 satisfy h.sub.1 &gt;h.sub.2, h.sub.1 =h.sub.2 and h.sub.1 &lt;h.sub.2 in the three areas, respectively. The iron core encloses the overall length of the coils. The superconducting coils generate a strong magnetizing force by which the magnetic poles are strongly saturated.
Thus, in the area of bending magnet where h.sub.1 &gt;h.sub.2 holds, the bending magnetic field is stronger on the outer circumference side than on the inner circumference side to produce a magnetic field which causes the charged particle beam to diverge in a direction perpendicular to the orbital plane of the charged particle beam. In the area where h.sub.1 &lt;h.sub.2 holds, the bending magnetic field is weaker on the outer circumference side than on the inner circumference side to produce a magnetic field which causes the charged particle beam to converge in the aforementioned direction. In the area where h.sub.1 =h.sub.2 holds, the magnetic field on the inner circumference side is equal to that on the outer circumference side and the bending magnetic field becomes uniform. Accordingly, the bending magnet per se is effective to converge or diverge the charged particle beam and is suitable for realization of a strongly focusing type synchrotron or storage ring removed of quadrupole magnet.
In the prior art, the vertical distance h.sub.1 between the inner circumference side coil segments is made to be equal to the vertical distance h.sub.2 between the outer circumference side coil segments for the purpose of obtaining the uniform bending magnetic field. However, since, in the prior art, magnetic saturation of the magnetic poles of the iron core was not fully taken into consideration, it was difficult to obtain sufficient uniformity of the magnetic field even if the coils were disposed to satisfy h.sub.1 =h.sub.2 upon detailed magnetic field calculation in consideration of non-linearity of iron core and experimental study. Thus, the prior art coil arrangement is unsuitable for the bending magnet. Especially, in a synchrotron or a storage ring in which the number of bending magnets is small, one bending magnet shares a large bending angle for the charged particle beam and the magnet configuration is sectoral or semi-circular, with the result that the non-uniformity of magnetic field is aggravated. Further, the prior art suggests a coil arrangement of making the vertical distance between inner circumference side coil segments different from the vertical distance between outer circumference side coil segments for causing the magnetic field to converge or diverge but nothing about improvement of uniformity of magnetic field. In conclusion, the prior art in no way takes into account improving the uniformity of magnetic field over the overall length of the orbit of charged particle beam in the bending magnet.
Japanese patent unexamined publications JP-A-62-186500 and JP-A-62-140400 also disclose a superconducting bending magnet, but none of these publications suggests anything about the above problem to be solved by the present invention.